Cortical representation of processing Japanese phonemic and phonetic contrasts

Abstract

This study investigated how Japanese speakers process phonemic and phonetic contrasts using voiced and devoiced vowel /u/ and /÷u/. During six oddball experiments, brain responses were measured using magnetoencephalography. Under the phonemic condition, a frequent stimulus /ch÷ita/ was contrasted with a deviant /ts÷uta/, and a frequent /ts÷uta/ with a deviant /ch÷ita/. Under the phonetic condition, a frequent /ts÷uta/ was contrasted with a deviant /tsuta/, and a frequent /tsuta/ with a deviant /ts÷uta/. Under the segment condition, vowel segments, /÷u/ and /u/, extracted from spoken words, were contrasted. The subjects were 13 native Japanese speakers. The equivalent current dipole moment (ECDM) was estimated from the mismatch field. Under the phonetic condition, the ECDM elicited by the voiced deviant was significantly larger than that elicited by the devoiced deviant in both hemispheres (p<0.01), while there were no significant deviant-related differences in ECDM under the phonemic condition in both hemispheres. Under the segment condition, the ECDM elicited by the voiced deviant and devoiced deviant did not differ significantly in either hemispheres. These results suggested that the ECDM asymmetries between the voiced and the devoiced deviant observed under the phonetic condition did not originate from the acoustical difference itself, but from the phonetic environment.